Signal Strength Calculator
The formula
What the received level tells you
Free-space signal level at a distance from a transmitter — the sanity check before blaming the vendor antenna.
Calculation: RSL = EIRP − FSPL(d, f); FSPL = 20log d + 20log f − 27.55. Enter your inputs and every substitution is shown below the result.
Reading the result
Real indoor paths sit 10–25 dB below free space from walls alone; if your survey beats this number you are seeing a reflection gain or the wrong EIRP.
Rules of thumb: Reference points: 10 m at 2.4 GHz from 20 dBm EIRP ⇒ ≈ −48 dBm; 100 m ⇒ ≈ −68 dBm.
Limitations and best practice
EIRP includes antenna gain — a "20 dBm" outdoor AP often radiates legally at 36 dBm EIRP with a dish; use the regulated figure.
This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Frequently asked questions
Why does my phone show −55 dBm at 5 m when this says −33?
Is this valid outdoors?
Source & methodology
Friis free-space loss formulation (H. T. Friis, 1925) in dB form.
Signal Strength: what this tool answers
You are here because you need a specific number, and you want to see how it was produced before you rely on it. There are plenty of pages about Signal Strength; almost none of them show the working. This one does: the formula, every input, a fully computed example, and a sensitivity analysis calculated live from this engine’s own defaults.
Like most useful measures, Signal Strength is a summary, and summaries hide as much as they reveal. The companion outputs and the input table are here to reopen what the headline compressed.
It helps to separate the mechanics from the interpretation. The mechanics are fixed: RSL = EIRP − FSPL(d, f); FSPL = 20log d + 20log f − 27.55. Anyone, anywhere, entering the same inputs will get the same -65.4 dBm. The interpretation is where experience counts: whether -65.4 dBm is good, normal or alarming depends on comparisons covered below, and that is the part most articles about Signal Strength never explain.
What makes a calculation page trustworthy is that its copy is derived from its own arithmetic: here received signal level = -65.4 dBm, recomputed every time a field changes, and the sensitivity section ranks the inputs that move the result — Transmitter EIRP first, at ≈2.6 % of the headline for a ten-percent error either way in that input. Read the sections in order and you will be able to reproduce every figure by hand.
How Signal Strength is computed, line by line
The engine implements a single stated model: RSL = EIRP − FSPL(d, f); FSPL = 20log d + 20log f − 27.55. There is no hidden coefficient and no login-gated precision — the formula printed is the formula executed, which is also what makes the result citable.
Transmitter EIRP (dBm) (example value 17) is a plain measured quantity — the unit shown in the label is the unit the formula expects. If you are unsure what belongs in this field, look at the printed steps: the substitution shows exactly how the engine uses it.
Distance (m) (example value 60) is a quantity rather than a category — precision here is cheap and worth having. Measure twice and use the unit the label asks for — length terms often scale the result geometrically.
Frequency (MHz) (example value 5,225) is a plain measured quantity — the unit shown in the label is the unit the formula expects. The example value is there to demonstrate the mechanics, not to describe your situation — overwrite it.
A fully worked Signal Strength example
Abstract formulas are hard to trust, so here is the complete substitution using the default inputs, written the way an examiner would want it. Starting figures: Transmitter EIRP (dBm), Distance (m), Frequency (MHz). Run to the end, the received signal level resolves to -65.4 dBm — Good — full rates at close range on the bands this tool uses.
- FSPL = 20·log(60 m) + 20·log(5,225 MHz) − 27.55
- FSPL = 82.4 dB
- RSL = 17 − 82.4 = -65.4 dBm
- Every 6 dB doubles or halves distance in this model
Free-space path loss — 82.4 dB. This is the number experienced users quote when the headline alone would mislead.
Bars-equivalent strength — 49.5%. If your inputs change, this is usually the first companion figure to move.
Margin above −75 usable floor — 9.6 dB. This is the number experienced users quote when the headline alone would mislead.
Distance at which −67 dBm edge is hit — 72.3 m. Read next to the headline, this is the figure that shows the shape of the trade-off.
Overwrite any input above and every line of this example updates with it — the substitution is live, not a screenshot of somebody else’s numbers.
The Signal Strength corridor: your result across a realistic band
Read this table as the practical error bar on the headline figure. It answers the question people actually have after seeing a single number: how wrong could this be if the main input is off by ten or twenty percent?
| Transmitter EIRP setting | Received signal level |
|---|---|
| 80 % of the example value | -68.8 dBm |
| 90 % of the example value | -67.1 dBm |
| example inputs | -65.4 dBm |
| 110 % of the example value | -63.7 dBm |
| 120 % of the example value | -62.0 dBm |
That spread — -68.8 dBm at the low end, -62.0 dBm at the high — is what a careful read of Signal Strength looks like: a band you can state out loud, with the reason for each edge. Reproduce it in the form above by editing Transmitter EIRP alone; the substitution steps will follow your figures exactly.
What actually moves your Signal Strength
This list was computed for this specific engine just now: each numeric input was nudged ±10 % while every other field held its example value, and the received signal level was recalculated from scratch. For Signal Strength the ranking is measured rather than guessed, and it belongs to these defaults: shift the starting point and the order can shift with it.
- Nudge Transmitter EIRP by ±10 % and Signal Strength lands between -67.1 dBm and -63.7 dBm (sensitivity ≈2.6 %)
- Frequency +10 % → Signal Strength = -66.2 dBm · −10 % → -64.5 dBm (the received signal level shifts ≈1.4 %)
- Distance +10 % → Signal Strength = -66.2 dBm · −10 % → -64.5 dBm (the received signal level shifts ≈1.4 %)
Transmitter EIRP is the dominant lever here: ten percent either way moves the result by about 2.6 %, from -67.1 dBm to -63.7 dBm. That is where a careless figure does the most damage, so check it against a source, and treat Frequency as the second priority (≈1.4 % per ten percent). The remaining fields can sit at safe defaults without materially changing the answer — knowing which is which is what separates a careful calculation from an anxious one.
What a Signal Strength of -65.4 dBm means in practice
The value above only becomes useful the moment you know what it is being measured against. The verdict attached to the default calculation — “Good — full rates at close range” — is one band on a ladder your own figure may sit somewhere else on. How much of that band is luck? On Transmitter EIRP alone ten percent either way covers -67.1 dBm to -63.7 dBm. The practical reading rules: ask what would have to change to move you one band. If the answer is “a small, plausible input difference”, the band itself is not the finding — the uncertainty is.
Where Signal Strength sits next to similar measures: this tool answers “how much is the Signal Strength”, while the related calculators answer “what do I do about it”. Reading the result alone tends to over-weight a single snapshot; pairing it with the companion tools above turns the number into a decision.
How people get Signal Strength wrong
EIRP includes antenna gain — a "20 dBm" outdoor AP often radiates legally at 36 dBm EIRP with a dish; use the regulated figure. This calculator gives an estimate based on standard formulas; it is not professional financial, medical, legal or engineering advice. Verify anything you plan to act on with a qualified professional and your own records.
Confusing a large number with an important one. Signal Strength can be dominated by one input whose absolute value looks unimpressive, which is why this page ranks influence separately from magnitude.
Attention is also misallocated between fields. Having measured this engine, we can say where it belongs: Transmitter EIRP first, everything else second.
Another is comparing across contexts: a Signal Strength computed with one set of conventions should never be lined up against a figure computed with another. Check the inputs behind both before ranking them.
Before you trust your Signal Strength: a short checklist
- Verify each input against its original source document, not memory.
- Re-run the calculation with a ±10 % stress on the two most sensitive inputs.
- Check Transmitter EIRP before anything else: it is the measured dominant input, ≈2.6 % of Signal Strength per ten percent.
- Confirm the example values are plausible for your own situation before replacing them.
- Compare the output against one published benchmark or a colleague’s independent estimate.
- Confirm the answer against one independent method or published benchmark for Signal Strength.
Five steps from raw inputs to a figure you can defend
Using a calculator well sounds trivial, but the difference between a decision-grade figure and a throwaway one lives entirely in the routine. The routine for Signal Strength takes a minute to learn and applies to every input above.
Step 1 — one document, one pass. Open the statement, label or record that holds transmitter eirp (dbm), distance (m), frequency (mhz) and transcribe straight into the form. Transcription from a single source removes the mismatch errors that appear when figures are assembled from three different places. If the verdict changes with a ten-percent nudge, report the nudge alongside the number.
Step 2 — baseline before exploration. Get one defensible Received signal level on screen before you start nudging numbers. Without a baseline, every variation looks interesting and none of them means anything. If the verdict changes with a ten-percent nudge, report the nudge alongside the number.
Step 3 — isolate your variables. Change one field, observe, revert. The live sensitivity list does the ranking for the default inputs; verify it against your own, because dominance shifts when the starting point shifts. If the verdict changes with a ten-percent nudge, report the nudge alongside the number.
Step 4 — stress before you commit. A decision that survives a ten-percent squeeze on its two biggest inputs needs no luck to be sound. One that does not survive is a decision about hope, not about arithmetic. A result worth acting on survives having its two shakiest inputs halved.
Step 5 — save the story with the figure. A number without its inputs and its date is unusable within a quarter. Record -65.4 dBm, the assumptions behind it, and today’s date; your future self will be auditing a stranger’s arithmetic. When in doubt, prefer the boring input: the interesting one is usually the one you misread.
From calculation to decision: where Signal Strength earns its keep
In Networks & Storage the figure computed here is usually the first one somebody asks to see. Having it — with the formula behind it — changes your position in the conversation from participant to reference. When in doubt, prefer the boring input: the interesting one is usually the one you misread.
The companion outputs — Free-space path loss and Bars-equivalent strength — exist for the second stage of that screen: the headline sizes the prize, while the companions describe the trade-off you are accepting to get it. Estimates improve most when the worst case is written down in ink.
Treat Signal Strength like a smoke alarm rather than a painting — its value is in being tested periodically, not in being admired once. When two sources disagree, it is almost never the arithmetic that differs.
Quick answers: what searchers ask about Signal Strength
What affects Signal Strength the most?
Measured on this engine, Transmitter EIRP is the dominant lever: moving it ±10 % shifts the received signal level across -67.1 dBm to -63.7 dBm — about 2.6 % of the headline, with Frequency next at ≈1.4 %. The ranked list in the sensitivity section is computed live, not asserted from a template.
Which matters more: frequency or transmitter eirp?
For Signal Strength, Transmitter EIRP wins the head-to-head (≈2.6 % against ≈1.4 % per ten-percent change). Knowing which input dominates is the difference between careful work and merely slow work.
What is a typical Signal Strength?
With the example inputs on this page the result is -65.4 dBm, labelled “Good — full rates at close range”. “Typical” varies with situation, which is why every result here ships with its verdict band and its sensitivity list: they turn one figure into a range you can locate yourself inside.
How do I calculate Signal Strength by hand?
Three steps: write the formula (RSL = EIRP − FSPL(d, f); FSPL = 20log d + 20log f − 27.55), insert your values, and simplify. The worked example section shows exactly that chain for the default inputs; replicating it once on paper teaches you more about Signal Strength than any number of calculator runs.
What happens to Signal Strength if transmitter eirp changes by 10 %?
Try it in the form above and the printed substitution will follow you; the pre-computed answer for the defaults is -67.1 dBm to -63.7 dBm across the ten-percent probe, i.e. ≈2.6 % sensitivity on the received signal level.
Is the Signal Strength result of -65.4 dBm good or bad?
The tool answers “what”, not “whether”: -65.4 dBm carries the verdict “Good — full rates at close range” for the default case, which is a starting interpretation rather than a verdict on your situation. Re-run with your own figures and compare the bands.
How do I calculate Signal Strength manually?
Write down the inputs, then follow the printed steps in order — RSL = EIRP − FSPL(d, f); FSPL = 20log d + 20log f − 27.55. A spreadsheet replicates it with two or three cells. The manual route is worth doing once to build intuition; after that the calculator saves time and eliminates transcription slips.
Why is my measured value different from Signal Strength?
Real hardware and networks add contention, throttling, and overheads that first-principles Signal Strength assumes away. Expect the measured number to sit below -65.4 dBm; if it sits far below, that gap itself is the diagnostic.
Why does my phone show −55 dBm at 5 m when this says −33?
Phone antennas are small and detuned by hands; a 10–20 dB client-side penalty against the calculator is normal.
Is this valid outdoors?
Yes — free space is the outdoor model; add roughly 6 dB per occupied building between AP and client.
Is my data private on this page?
Yes — computation happens locally in your browser, with no account, no tracking of your inputs and nothing sent to a server. The printed formula is the whole story of what happens to your figures.
Glossary for Signal Strength
- Free-space path loss
- 82.4 dB for the Signal Strength worked case; it sits next to the main result because it carries context the headline leaves out.
- Bars-equivalent strength
- 49.5% in the default Signal Strength calculation — one of the secondary figures an experienced reader checks before trusting the primary number.
- Margin above −75 usable floor
- 9.6 dB for the Signal Strength worked case; it sits next to the main result because it carries context the headline leaves out.
- Received signal level
- The primary output of this tool: -65.4 dBm for the default inputs, computed exactly from RSL = EIRP − FSPL(d, f); FSPL = 20log d + 20log f − 27.55.
- Worked example
- The calculation written out in full so that nothing on this page has to be taken on trust.
- Verdict band
- The plain-language label — here “Good — full rates at close range” — mapping the computed figure onto experience-based ranges for Technology & Computing.
- Sensitivity (±10 %)
- How far the Signal Strength moves when a single input is nudged ten percent either way; the ranked list on this page is computed live from this engine.
Method, sources and what this tool cannot tell you
Everything above runs on one stated formula, applied without hidden coefficients: RSL = EIRP − FSPL(d, f); FSPL = 20log d + 20log f − 27.55. Source of record: Friis free-space loss formulation (H. T. Friis, 1925) in dB form. Any other implementation of this model should return the same figure for the same inputs. If one does not, the difference is in its assumptions — which this page prints and that one probably does not.
Treat the output as a precise answer to the question you typed: exact arithmetic, transparent formula, published source, no hidden rounding. What that buys you is reproducibility — two people, two browsers, the same arithmetic, and a printed trail from inputs to result that either of them can audit.
After the number: what to do with it
Save the number together with its inputs and today’s date; a Signal Strength figure without context is unusable the moment an input changes.
This page was written for people who need one trustworthy number before their next decision, and the promise is that nothing above required trust. Bookmark the tool, re-run it whenever an input moves by more than a few percent, and keep the date with the number.